Tony Buckingham, the buccaneering multimillionaire oilman, told a tribunal he would not bend his evidence to protect his "great mate" Ian Hannam, the City of London dealmaker who is fighting a £450,000 market abuse fine.

Buckingham, founder and chief executive of Heritage Oil, took the stand on the third day of the hearing. Hannam is accused of passing on insider information.

The case mounted by the Financial Conduct Authority, formerly the Financial Services Authority, turns on two 2008 emails that Hannam sent about Heritage Oil, his longstanding client, to Ashti Hawrami, the oil minister of the Kurdistan Regional Government in northern Iraq, who was a potential client of Hannam and investor in Heritage.

One email said that Heritage had made an oil discovery that was "looking good" – several weeks before this market sensitive information was publicly released. The FCA states it would have been clear from public information that the email related to Heritage's Warthog-1 well in Uganda.

The FCA's counsel said Buckingham's "first instinctive reaction" was surprise when confronted with that October email by the FSA at an interview five years ago, whereas in the tribunal his "generally honest evidence" was tinged with "a degree of rationalisation".

Buckingham said he would not bend the evidence to help his old friend. "Although he is a great mate and I think the world of him, would I go out of my way to manipulate the evidence? Absolutely not.

"I felt there was an implicit understanding between the parties and the implicit understanding was that everyone was aware of what sensitive information was," he said.

Buckingham said that the postscript – "Tony has just found oil and it is looking good" – was wrong. "The reference to 'we have found oil' was clearly wrong. I certainly would not have been in a position to say definitively 'we've found oil'."

As well as Buckingham's assessment of Hawrami, whom he described as "a very competent wily old fox", the tribunal also shed light on deal-making in the oil world. Buckingham recounted his fury when the privately owned French oil company Perenco made a £400m offer for Heritage – less than half what he had hoped for.

"It was a beautiful sunny day and I was walking in Verviers in the mountains," he told the tribunal of the moment when the ill-fated phonecall came through. "I can't remember the exact expletive I used but I was none too pleased." The talks had been "a complete waste of everybody's time" and "a fishing exercise," he said.

In his testimony, Hannam told the tribunal he had not passed on insider information and said some of his words were being taken out of context by the FCA counsel.